A conventional camshaft support structure used for an internal combustion engine for a car and a two-wheeled motor vehicle is disclosed in Japanese Unexamined Patent Publication No. 2005-90696, for example. Referring to FIG. 44, the camshaft support structure described in the document comprises a camshaft 201 having a cam lobe 201a, a cylindrical journal part 201b supported by a roller bearing 202, and an end large diameter part 201c, a housing comprising a cylinder head 208 and a cap 209, and the roller bearing 202 including a plurality of rollers 203, roughly semi-cylindrical retainers 204 and 205, and roughly semi-cylindrical race plates 206 and 207, and supporting the camshaft 201 rotatably with respect to the housing.
Here, the outer diameter dimension of the journal part 201b is smaller than the maximum dimension of the outer diameter of the cam lobe 201a and the outer diameter dimension of the end large diameter part 201c. Therefore, the roller bearing 202 to be arranged at the journal part 201b to support the camshaft 201 rotatably cannot be inserted from the axial direction of the camshaft 201.
Thus, the roller bearing 202 has the plurality of rollers 203, the circumferentially split roughly semi-cylindrical retainers 204 and 205, and the circumferentially split roughly semi-cylindrical race plates 206 and 207 arranged between the cylinder head 208 and the cap 209. In addition, referring to FIG. 45, the race plate 207 has two projections 207a projecting from the circumferential each end to the radial outer side, and the cap 209 has recessions 209a corresponding to the projections 207a. 
Thus, according to the document, when the projections 207a engage with the recessions 209a, the relative movement between the race plate 207 and the cap 209 is prevented in the circumferential direction and an axial direction during the rotation of the roller bearing 202. In addition, the same is true on the space between the race plate 206 and the cylinder head 208.
The race plates 206 and 207 having the above constitution are produced by pressing a steel plate such as cold rolled steel plate (SPC) in general. In addition, a heat treatment is performed in order to obtain predetermined mechanical property such as hardness after the predetermined configuration is provided.
According to the roller bearing 202 disclosed in the above document, the projection 207a is formed by applying force in the direction of radial outer side from the inner diameter surface of the race plate 207 so that the outer diameter surface thereof is protruded. As a result, a recession is formed in the inner diameter surface of the race plate 207 serving as a track surface of the roller 203, which causes vibration to be generated when the roller 203 passes through the recession, and the surface of the roller 203 to be abraded in an early stage, so that the smooth rotation of the roller 203 is hindered.
Furthermore, although the lubricant oil flows into the roller bearing 202 from an oil hole (not shown) provided in the race plates 206 and 207 or an oil hole (not shown) provided in the camshaft 201, the flow of the lubricant oil in the bearing is interrupted by the retainers 204 and 205 and the problem is that the lubricant oil cannot be uniformly supplied to the whole bearing.
In addition, this problem arises in the bearing supporting the crankshaft and the rocker shaft as well as the roller bearing that supports the camshaft.
In addition, a lubricating structure around the camshaft 201 described above is disclosed in Japanese Unexamined Patent Publication No. 2000-110533, for example. When the lubricating structure disclosed in this document is described with reference to FIG. 44, the camshaft 201 comprises an oil passage (not shown) extending in an axial direction, and a plurality of oil holes (not shown) extending from the oil passage to the surface of the journal part 201b. Meanwhile, a housing comprises a plurality of supply oil paths (not shown) through which lubricant oil is supplied from the outside to the position opposed to the journal part 201b and an oil groove (not shown). Thus, the lubricant oil supplied from the supply oil path provided in the housing is distributed to each part through the oil groove, the oil hole and the oil passage of the camshaft 201.
According to the lubricating structure disclosed in the Japanese Unexamined Patent Publication No. 2000-110533, when the oil groove is formed in the housing by cutting process, the number of production steps and production cost of the internal combustion engine are increased. This problem is serious in the engine having many cylinders. In addition, when the housing is produced by casting, deformation of a die due to heat becomes a serious problem. This will cause an error in the position and the configuration of the oil groove.
In addition, according to the above camshaft support structure, since the gap is formed between the camshaft 201 and the axial ends of the race plates 206 and 207, a part of the lubricant oil supplied from the oil supply path of the housing into the roller bearing 202 flows out through the gap and the amount of the lubricant oil reaching the oil hole of the camshaft 201 is reduced. As a result, since the amount of the lubricant oil distributed to each part through the oil passage is also reduced, a lubricating property is lowered as a whole.
In addition, since a load slanted to a certain direction is applied to the camshaft 201 at the time of rotation, the camshaft 201 is divided to a region in which a relatively large load (referred to as “load region” hereinafter) and a region in which only a relatively small load is applied (referred to as “non-load region” hereinafter) in its circumferential direction. Therefore, it is desirable that the lubricant oil supplied from the supply oil path of the housing is supplied more to the load region.